Bug Asset Bundles and/or build pipeline cause shader recompiles far too often

I've been trying to optimise our build pipeline recently and I'm really struggling with how unreliable the Build Pipeline and/or the Asset Bundle system appears to be. I am finding that sometimes subsequent builds on the exact same assetbases, even minutes apart, can cause lengthy bundle rebuilds which in turn seem to cause shaders in those bundles to be recompiled (or maybe it is them causing the AB rebuilds?). On the other hand, sometimes it works fine and our builds take minutes instead of hours.

What's worst is that I can't find a way to inspect the process further. What assets are "dirty" that cause Unity to think an AB needs to be rebuilt? If I could get access to more data I would happily dive futher into root causes.